A 67-year-old man with COPD presents with increasing dyspnoea and sputum production over 3 days. On examination, he is using accessory muscles and has a respiratory rate of 26 breaths/min. Arterial blood gas on room air: pH 7.32, PaCO₂ 7.2 kPa, PaO₂ 7.8 kPa, HCO₃⁻ 30 mmol/L. According to COPD-X and emergency guidance, what oxygen saturation range should be targeted initially with controlled oxygen therapy?